In the art of the automatic focusing of cameras, the problem associated with low contrast conditions in the scene being viewed has long been recognized. If the scene being viewed has too low a contrast, then the output signals from the detectors used in the auto focus signal become unreliable for purposes of properly positioning the camera lens at a correct focus position. Accordingly, it has been desired in the past to detect low contrast conditions and provide a means for default when such conditions exist. The default signal either informs the operator that a low contrast condition exists and/or positions the taking lens at the hyperfocal position which position causes a proper focus for many scenes being taken or just inhibiting lens motion.
A common method used for low contrast default is to provide a bias signal set at some predetermined value below which the contrast conditions are assumed to be too low for proper operation. A signal from the auto focus system, which is representative of the actual contrast being viewed, is compared with the bias signal and if the contrast signal falls below the bias signal, the system is caused to default. Examples of prior art default systems may be found in my U.S. Pat. No. 4,247,762 issued Jan. 27, 1981 and U.S. Pat. No. 4,250,377 issued Feb. 10, 1981 as well as in copending applications of James Frazier, Ser. No. 212,930 filed Dec. 4, 1980, now U.S. Pat. No. 4,358,674, of John C. Wittenberger, Ser. No. 212,918 filed Dec. 4, 1980, now U.S. Pat. No. 4,358,673, and of James Frazier et al, Ser. No. 213,437 filed Dec. 5, 1980, now U.S. Pat. No. 4,358,675, all of which are assigned to the assignee of the present invention.
A hitherto unidentified problem occurs in some auto focus systems, particularly those of the "through-the-lens" type, such as is disclosed in a copending application of Richard Langlais et al, Ser. No. 168,225 filed July 10, 1980, now U.S. Pat. No. 4,333,007, and assigned to the assignee of the present invention. In this system, the taking lens of the camera is used to direct radiation to the auto focus sensing circuitry and its position at the time the auto focus signal is determined may be near the proper focus position or quite remote from the proper focus position. When the lens is remote from the proper focus position, the radiation received by the detectors is considerably out of focus and thus inherently provides a rather low contrast image for the detectors to sense. The image falling on the detectors when the lens is near the proper focus position is not so much out of focus and thus produces a greatly increased indication of contrast. Accordingly, if a constant bias signal is used as a gauge to determine whether a low contrast condition exists, the system may default when the lens is far out of the normal focus position even though sufficient contrast may exist when the lens nears the proper focus position. Thus, the low contrast default systems of the prior art might frequently produce a default signal when it was not actually necessary.